An aminium salt and a diimmonium salt are useful as near-infrared absorbing dyes that do not substantially absorb visible light but absorb infrared rays, and they have been studied enthusiastically (for example, JP-A-2003-280247 (“JP-A” means unexamined published Japanese patent application), JP-A-2003-295496, and JP-A-2004-145036).
As a method of producing an aminium salt or a diimmonium salt, methods in which an amino compound, that is a precursor, is oxidized by Cu2+ (for example, JP-B-59-40825 (“JP-B” means examined Japanese patent publication) and JP-A-63-51462); by Fe3+ (for example, JP-A-2-311447 and JP-A-11-315054); by utilizing an oxidizing reaction using a solid catalyst (for example, JP-A-5-98243); by a peroxodisulfate (for example, JP-A-2003-55643); by using silver hexafluoroantimonate (for example, Journal of Dispersion Science and Technology, vol. 23, p 555 (2002)), or by electrical oxidation (for example, JP-A-61-246391), have been known so far. All of these methods are unsatisfactory in yield, and they have also a large environmental load because of the use of heavy metal ions. These methods also have such problems as high costs.